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    SubjectRe: [patch 4/21] fix ARCH_HAS_PREFETCH
    On Wed, Aug 14, 2002 at 12:41:04PM -0700, H. Peter Anvin wrote:
    > Rogier Wolff wrote:
    > >>
    > >>Bullsh*t. It can legitimately transform it into:
    > >>
    > >> i = N;
    > >
    > >
    > >Right! But people are confusing "practise", "published interface", and
    > >"spec" again.
    > >
    > >Published interface in this case is that gcc will not optimize an empty
    > >loop away, as it is often used to generate a timing loop.
    > >
    >
    > Yes. This is a gcc-specific wart, a bad idea from the start, and
    > apparently one which has caught up with them to the point that they've
    > had to abandon it.

    There would be a solution to tell gcc not to optimize things, which may
    not require too much work from gcc people. Basically, we would need to
    implement a __builtin_nop() function that would respect dependencies but
    not generate any code. This way, we could have :

    for (i=0; i<N, i++);

    optimized as i=N
    and
    for (i=0; i<N; i++)
    __builtin_nop();
    or even
    for (i=0; i<N; __builtin_nop(i++));
    do the real work.

    This way, some loops could be optimized, and the developpers could explicitely
    tell the compiler when they need to prevent any optimization.

    Cheers,
    Willy

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